Consumer products such as toilet tissue, toweling and facial tissue made from cellulosic webs are a pervasive part of modern society. In general, these products need to possess certain key physical properties to be considered acceptable to consumers. While the exact mix of key properties and the absolute value of the individual properties will vary depending on the nature of the product, nonetheless, softness, wet and dry strength, absorbency, and pleasing aesthetic nature are universally desirable properties. Softness is that aspect of the fibrous web that elicits a pleasing tactile response and insures that the product is not harsh or abrasive when it contacts human skin or other fragile surfaces. Strength is the ability of the structure to retain its physical integrity during use. Absorbency is the property of the fibrous structure which allows it top acquire and retain contacted fluids in an acceptable time. Aesthetic nature refers to the psycho-visual response that occurs when the consumer views the product either alone or in the context of the product's surroundings.
The most common method for the manufacture of tissue products is the wet laid papermaking process. In such a process, individual fibers are first suspended in a dilute slurry with water. This slurry is then laid on a foraminous to remove a large portion of the water and to form a thin, relatively uniform-weight embryonic web. This embryonic web is then molded and/or dried in a variety of ways to form the final tissue web. As part of this process the molded and/or dried web is usually glued to a drying drum and subsequently creped from the surface of the dryer to impart desirable properties.
Products made by many existing wet laid processes fall under the above description. Examples of such webs that are soft, strong, and absorbent and contain at least two micro regions of density can be found in, U.S. Pat. Nos.: 3,301,746 which issued Jan. 31, 1967, to Lawrence H. Sanford and James B. Sisson; 3,974,025 which issued Aug. 10, 1976, to Peter G. Ayers; 3,994,771 which issued Nov. 30, 1976, to George Morgan, Jr. and Thomas F. Rich; 4,191,609 which issued Mar. 4, 1980, to Paul D. Trokhan; and 4,637,859 which issued Jan. 20, 1987, to Paul D. Trokhan. Each of these papers is characterized by a repeating pattern of dense areas and less dense areas The dense areas can be either discrete or continuous. These dense areas result from localized compaction of the web during papermaking by raised areas of an imprinting carrier fabric or belt.
Other high-bulk, soft tissue papers are disclosed in U.S. Pat. No. 4,300,981 which issued Nov. 17, 1981, to Jerry E. Carstens; and 4,440,597 which issued Apr. 3, 1984, to Edward R. Wells and Thomas A. Hensler.
Additionally, achieving high-bulk, soft and absorbent tissue paper through the avoidance of overall compaction prior to final drying is disclosed in U.S. Pat. No. 3,821,068 which issued Jun. 28, 1974, to D. L. Shaw; and avoidance of overall compaction in combination with the use of debonders and elastomeric bonders in the papermaking furnish is disclosed in U.S. Pat. No. 3,812,000 which issued May 21, 1974, to J. L. Salvucci, Jr.
Chemical debonders such as those contemplated by Salvucci, referred to above, and their operative theory are disclosed in such representative U.S. Pat. Nos. as 3,755,220 which issued Aug. 28, 1973, to Friemark et al.; 3,844,880 which issued Oct. 29, 1974, to Meisel et al.; and 4,158,594 which issued Jan. 19, 1979, to Becker et al.
Tissue paper has also been treated with cationic surfactants, as well as noncationic surfactants to enhance softness. See, for example, U.S. Pat. No. 4,959,125 which issued Sep. 25, 1990, to Spendel; and U.S. Pat. No. 4,940,513 which issued Jul. 10, 1990, to Spendel, that disclose processes for enhancing the softness of tissue paper by treating it with noncationic, preferably nonionic, surfactants.
It has been found that the softness of tissue paper, in particular, high-bulk pattern densified tissue papers, can be improved by treatment with various agents such as vegetable, animal or synthetic oils, and especially polysiloxane materials typically referred to as silicone oils. See, for example, U.S. Pat. No. 5,059,282 which issued Oct. 22, 1991, to Ampulski et al. The Ampulski patent discloses a process for adding a polysiloxane compound to a wet tissue web (preferably at a fiber consistency of between about 20% and about 35%). These polysiloxane compounds impart a silky, soft feeling to the tissue paper.
While the processes described above generally make acceptable product properties, the product properties can be further enhanced. However, processes to make current products and potentially enhanced products suffer from several drawbacks. For example, the chemicals used to strengthen tissue webs are often added to the dilute slurry of water and fibers prior to the initial lay down on the forming screen. This is a relatively convenient and cost effective way to introduce additives. However, other chemicals to aid absorbency or to improve softness are also commonly added to the so called wet end of the tissue making process. Because of the complex nature of the individual chemicals used to generate the key properties, they often interact with each other in an adverse manner. They can compete with each other for the desired retention on the cellulose fibers as well as destroy properties that are inherent in the fibers. For example softening chemicals often reduce the natural tendency of fibers to bond to other fibers and hence reduce the functional strength of the resulting web. Both the process and the product benefit if the chemical papermaking additives introduced in the wet end are kept to a minimum.
As previously mentioned, the majority of the existing tissue manufacturing processes glue the web to the surface of a drying drum and subsequently crepe the web from the dryer surface Creping generally produces a web with improved softness and importantly improves the extensibility of the web. For proper creping to occur, it is imperative that the web be securely attached to the surface of the drum. Many of the chemicals added to the wet end of the machine, to ostensibly improve key properties, end up interfering with the adhesion of the web to the drying drum and hence adversely affect the creping process and the quality of the tissue produced. The creping operation runs optimally when the adhesive used to adhere the web to the creping surface is free of interference from non-creping related chemicals such as those added at the wet end of the overall tissue making process.
Additives introduced in the wet end of the process must be retained by the cellulose fibers if the chemicals are to be functional. This is generally done by using chemicals that possess an ionic charge; most preferably a positive ionic charge which is attracted to the inherent negative ionic charge of cellulose. Many additives which could improve the properties of the web are not charged. Introduction of such chemicals into the dilute fiber slurry at the wet end of the process results in poor retention and exacerbates the interference problems described above.
Another drawback to adding any chemical to the wet end of the process is that the chemical, if retained, is distributed throughout the web. In many instances it is desirable to apply active ingredient(s) only to the surface of the web. This may, for instance, be desirable with lubricious softening materials. Application only to the surface insures efficient use of the material since consumers only tactically interact with the surface. Application to the surface also avoids interference with other materials, such as strength additives, that might best be included in the center of the sheet. The present invention overcomes all of the above mentioned drawbacks and generates desirable additional benefits.
It is therefore, an object of this invention to provide an improved process to incorporate chemical papermaking additives into the tissue web that enhance softness, strength, absorbency, and aesthetics or combinations of these properties.
It is a further object of this invention to provide an improved process to incorporate chemical papermaking additives into the tissue web that enhance softness, strength, absorbency, and aesthetics, or combinations of these properties, without interference with the creping operation or disruption of the delicate water system balance or loss of beneficial properties generated by other means.
It is a further object of this invention to provide an improved process to incorporate chemical papermaking additives into the tissue web that are typically poorly retained when added at the wet end of the papermaking process.
It is a further object of this invention to provide a process for adding chemical papermaking additives to the dry web at the calender stack.
It is a further object of this invention to provide an improved process to apply diluted chemical papermaking additives (diluted to insure controlled application of small quantities of additive) to a heated transfer surface, to preferentially evaporate the solvent or carrier material while the mixture is on the transfer surface but prior to addition to the dry web and subsequently to apply a more concentrated mixture of the additive and solvent to the surface of the tissue web than was initially applied to the transfer surface.
It is a further object of this invention to provide an improved process to apply chemical papermaking additives to the tissue web via the process described above where the vapor pressure of the carrier or solvent material is higher than that of the additive material such that the carrier is preferentially depleted after application to the heated transfer surface. Preferably this carrier depletion also occurs prior to application to the tissue web.
These and other objects are obtained using the present invention, as will be seen from the following more detailed disclosure.